A method for measuring the partial pressure of a specific gas in a gas to be measured that has been proposed in recent years is laser absorption spectroscopy that uses a wavelength variable laser (see, for example, Patent Literature 1). With this method, a sample cell into which a gas to be measured has been introduced is irradiated with a laser beam while changing its wavelength within a predetermined wavelength range. The laser beam that has been transmitted through the gas to be measured is analyzed, and a partial pressure of a specific gas is calculated from the amount of absorption by the specific gas in the gas to be measured. Because there is no contact between the gas to be measured and the light source or the photoreceptors, this device offers advantages such as measurements being possible without disrupting the field (non-contact measurement) and the extremely quick response time (high time-resolution).
FIG. 1 shows one example of a gas analyzer of the previous type that uses laser absorption spectroscopy. With this gas analyzer, a sample cell 1 is positioned to be substantially orthogonal to the direction of gas flow path 2 through which the gas to be measured flows. Reflection mirrors 3 and 4, which oppose each other, are disposed at either ends of sample cell 1. A transparent window 5 through which only light can pass is disposed on reflection mirror 3. An optical chamber 6 with a substantially sealed structure and whose atmosphere is at substantially atmospheric pressure is disposed outside of sample cell 1 with said transparent window 5 interposed in between. Contained within the optical chamber 6 are a wavelength variable laser 7 and a photodetector 8.
A pressure sensor 9 and a gas temperature sensor 10 are installed in sample cell 1, the pressure sensor 9 measuring the pressure (total pressure) of the gas to be measured and the gas temperature sensor 10 measuring the temperature of the gas to be measured.
With the gas analyzer shown in FIG. 1, under the control of laser controller 11, wavelength variable laser 7 emits a laser beam while varying its wavelength within a predetermined wavelength range that includes the center wavelength of the absorption spectrum of a specific gas. The laser beam that is emitted by wavelength variable laser 7 passes through transparent window 5 and enters sample cell 1 and makes round-trips between reflection mirrors 3 and 4 during which the laser beam is absorbed by a specific gas in the gas to be measured. The laser beam then travels through transparent window 5 and returns into optical chamber 6 where the intensity of the transmitted light is measured by photodetector 8.
The transmission light intensity measured by photodetector 8 and the pressure and the temperature of the gas to be measured—measured by pressure sensor 9 and gas temperature sensor 10—undergo signal processing such as analog/digital conversion and noise elimination and are input to computation unit 12 where the partial pressure of the specific gas is calculated using a predetermined computational process.    Patent Literature 1: Unexamined Patent Application Publication No. H05-99845    Non-Patent Literature 1: J. J. Olivero and R. L. Longbothum, “Empirical Fits to the Voigt Line Width: A Brief Review,” Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 17 (1977), pp. 233-236.